1. Field of the Invention
This invention relates to a process and apparatus for regenerating fluidized cracking catalyst. More particularly, it relates to a process and apparatus including staged regeneration and separation of flue gas from catalyst particles to minimize or substantially eliminate hydrothermal deactivation.
2. Discussion of the Prior Art
The field of catalytic cracking, and particularly fluid catalyst operations, has undergone significant development improvements due primarily to advances in catalyst technology and product distribution obtained therefrom. With the advent of high activity catalyst and particularly crystalline zeolite cracking catalysts, new areas of operating technology have been encountered, requiring even further refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
By way of background, the hydrocarbon conversion catalyst usually employed in a fluid catalytic cracking (FCC) installation is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed phase condition generally upwardly through one or more riser conversion zones (fluid catalytic cracking zones), providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature riser hydrocarbon conversions, occurring at temperatures of about 1000.degree. F. or higher and at 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the riser, are desirable for some operations before initiating separation of vaporous hydrocarbon materials from the catalyst. Rapid separation of catalyst from hydrocarbons discharged from a riser conversion zone is particularly desirable for restricting hydrocarbon overcracking.
Closed cyclone systems have been employed in fluid catalytic cracking reactor vessels to quickly separate hydrocarbon product from cracking catalyst, thereby preventing overcracking. Closed cyclone systems are further discussed in U.S. Pat. No. 4,502,947 to Haddad et al, herein incorporated by reference.
During the hydrocarbon conversion step, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion step. The entrained hydrocarbons are subjected to further contact with the catalyst until they are removed from the catalyst by stripping gas in a separate catalyst stripping zone. Hydrocarbon products separated from the catalyst and stripped materials are combined and typically passed to a product fractionation step. Stripped catalyst (spent catalyst) containing deactivating amounts of carbonaceous material, hereinafter referred to as coke, is then passed to a catalyst regeneration operation.
In catalyst regeneration, the spent catalyst contacts oxygen to burn off coke. However, spent catalyst contains hydrogen-containing components, such as coke, adhering thereto. This causes hydrothermal degradation because the hydrogen reacts with oxygen in the regenerator to form water.
U.S. Pat. No. 4,336,160 to Dean et al attempts to reduce hydrothermal degradation by staged regeneration. However, the first stage of the regeneration process of Dean et al employs a dense bed which provides an opportunity for hydrothermal deactivation.
Fluid catalytic cracking regenerators have been developed which employ a fast fluid bed riser. Such a fast fluid bed is disclosed in U.S. Pat. No. 4,444,722 to Owen, herein incorporated by reference.
Both intermediate pore zeolites, such as ZSM-5, and large pore zeolites, such as zeolite Y, are employed for fluid catalytic cracking. Combinations of ZSM-5 and other zeolites, such as zeolite Y, have been disclosed in U.S. Pat. No. 3,758,403 to Rosinski et al, herein incorporated by reference.
Intermediate pore zeolites, particularly ZSM-5, do not coke up as rapidly do the large pore zeolites, for example, zeolite Y. Therefore, ZSM-5 requires less regeneration than zeolite Y.
It would be desirable to minimize hydrothermal degradation. It would further be desirable to employ a combination of ZSM-5 catalyst particles and zeolite Y catalyst particles in a process which does not regenerate the ZSM-5 catalyst to the extent which it regenerates zeolite Y containing catalysts.